This invention is directed to an improvement in spacer grids for nuclear reactor fuel assemblies.
Nuclear reactors for the production of power are, today, almost all provided with rod-type fuel elements containing uranium oxide pellets including fissionable isotopes, such as uranium-235 or uranium-233, and sometimes plutonium-239 oxide. These rods are in fuel assemblies, which comprise a large number of rods spaced from each other and held in position by suitable spacing means.
These spacing means ordinarily take the form of grids, such as are shown in U.S. Pat. No. 3,379,617, granted Apr. 23, 1968 to Harry N. Andrews and Herbert W. Keller. An improved form is shown in U.S. Pat. No. 4,077,843, granted Mar. 7, 1978 to John F. Patterson and Barney S. Flora. These grids may be formed of a single metal, such as stainless steel or one of the zircaloys, or, as shown in the Patterson and Flora patent, they may be made of two different metals. The common characteristic is that they are made up of a large number of thin metallic straps, or plates, which cross each other, usually at right angles, and contain springs which press against the fuel rods. The plates are positioned so that they present edges to the flow of coolant, which may be water or a liquid metal, such as molten sodium.
In the United States, the flow of liquid in power reactors is invariably vertical and in an upward direction. In other types of reactors used in other countries, the flow may be horizontal. The flow of coolant is at a high velocity, and the power required to pass it through the assemblies is considerable. An appreciable proportion of the resistance to flow in an assembly is caused by the spacer grids.